Avoiding Micro-loops in a Ring Topology of a Network

ABSTRACT

In one embodiment, micro-loops are avoided in ring topologies of packet switching devices by changing the order of propagation of link state information concerning failed communications between a particular packet switching device and a neighbor packet switching device. In one embodiment, the particular packet switching device communicates link state information of a high cost of the particular communications (e.g., in the direction from particular to neighbor packet switching devices) such that this link state information will propagate towards the particular packet switching device from at least from the furthest packet switching device in the ring topology that is currently configured to forward packets having a destination address of the neighbor packet switching device through the particular packet switching device.

TECHNICAL FIELD

The present disclosure relates generally to communicating information ina network including packet switching devices.

BACKGROUND

The communications industry is rapidly changing to adjust to emergingtechnologies and ever increasing customer demand. This customer demandfor new applications and increased performance of existing applicationsis driving communications network and system providers to employnetworks and systems having greater speed and capacity (e.g., greaterbandwidth). In trying to achieve these goals, a common approach taken bymany communications providers is to use packet switching technology.

One topology of a network including packet switching devices is a ringtopology, in which packet switching devices are communicatively coupledtogether to form the ring. Routing protocols are used to exchangeinformation for determining how each packet switching device shouldforward packets in the network. For example, a link-state routingprotocol is performed by each packet switching device, in which itbuilds a connectivity map of the network. Each packet switching deviceindependently calculates the next best logical path from itself to otherdestinations in the network based on the connectivity map and theassociated costs of the links communicatively coupling the packetswitching devices. The collection of best paths will then form thenode's routing table, which is used in the data plane to forwardpackets.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth the features of one or more embodimentswith particularity. The embodiment(s), together with its advantages, maybe best understood from the following detailed description taken inconjunction with the accompanying drawings of which:

FIG. 1 illustrates a network configured to operate, and/or operating,according to one embodiment;

FIG. 2 illustrates a packet switching device configured to operate,and/or operating, according to one embodiment;

FIG. 3 illustrates an apparatus or component configured to operate,and/or operating, according to one embodiment;

FIG. 4 illustrates a process configured to be performed, and/orperformed, in one embodiment; and

FIG. 5 illustrates different packets used to exchange link stateinformation in one embodiment.

DESCRIPTION OF EXAMPLE EMBODIMENTS 1. Overview

Disclosed are, inter alia, methods, apparatus, computer-storage media,mechanisms, and means associated with avoiding micro-loops in a ringtopology of a network.

One embodiment includes a network, comprising: a plurality of packetswitching devices communicatively coupled in a ring topology, with theplurality of packet switching devices including: a particular packetswitching device, a neighbor packet switching device, a second neighborpacket switching device, and an intermediate packet switching device;wherein the particular packet switching device and the neighbor packetswitching device are neighbors in the ring topology when particularcommunications is established between the particular packet switchingdevice and the neighbor packet switching device; wherein the particularpacket switching device and the second neighbor packet switching deviceare neighbors in the ring topology; wherein each of the plurality ofpacket switching devices are configured to exchange link stateinformation and to update their respective local forwarding informationaccordingly. In one embodiment, the particular packet switching deviceis configured to cause a particular link state packet, includingparticular link state information identifying a very high cost of theparticular communications for reaching the neighbor packet switchingdevice from the particular packet switching device, to be communicatedto the neighbor packet switching device such that said particular linkstate information is processed by the neighbor packet switching devicebefore any other packet switching device in the plurality of packetswitching devices; and wherein the particular packet switching device isconfigured not to flood said particular link state information to thesecond neighbor packet switching device.

One embodiment includes a network, comprising: a plurality of packetswitching devices communicatively coupled in a ring topology, with theplurality of packet switching devices including: a particular packetswitching device, a neighbor packet switching device, a second neighborpacket switching device, and an intermediate packet switching device;wherein the particular packet switching device and the neighbor packetswitching device are neighbors in the ring topology when particularcommunications is established between the particular packet switchingdevice and the neighbor packet switching device; wherein the particularpacket switching device and the second neighbor packet switching deviceare neighbors in the ring topology; wherein each of the plurality ofpacket switching devices are configured to exchange link stateinformation and to update their respective local forwarding informationaccordingly; wherein the intermediate packet switching device isconfigured to forward packets addressed to the neighbor packet switchingdevice to another packet switching device of the plurality of packetswitching devices towards the neighboring node and away from theparticular packet switching device in the ring topology, or is thefurthest packet switching device from the particular packet switchingdevice in the ring topology that is configured to forward packetsaddressed to the neighbor packet switching device through the particularpacket switching device. In one embodiment, the particular packetswitching device is configured to encapsulate a particular link statepacket, including particular link state information identifying a veryhigh cost of the particular communications for reaching the neighborpacket switching device from the particular packet switching device, andto send said encapsulated particular link state packet to theintermediate packet switching device in response to the particularcommunications being no longer available between the particular packetswitching device and the neighbor packet switching device; and whereinthe particular packet switching device is configured not to flood saidparticular link state information to the second neighbor packetswitching device.

2. Description

Disclosed are, inter alia, methods, apparatus, computer-storage media,mechanisms, and means associated with avoiding micro-loops in a ringtopology of a network. Embodiments described herein include variouselements and limitations, with no one element or limitation contemplatedas being a critical element or limitation. Each of the claimsindividually recites an aspect of the embodiment in its entirety.Moreover, some embodiments described may include, but are not limitedto, inter alia, systems, networks, integrated circuit chips, embeddedprocessors, ASICs, methods, and computer-readable media containinginstructions. One or multiple systems, devices, components, etc. maycomprise one or more embodiments, which may include some elements orlimitations of a claim being performed by the same or different systems,devices, components, etc. A processing element may be a generalprocessor, task-specific processor, or other implementation forperforming the corresponding processing. The embodiments describedhereinafter embody various aspects and configurations, with the figuresillustrating exemplary and non-limiting configurations. Note,computer-readable media and means for performing methods and processingblock operations (e.g., a processor and memory or other apparatusconfigured to perform such operations) are disclosed and are in keepingwith the extensible scope and spirit of the embodiments. Note, the term“apparatus” is used consistently herein with its common definition of anappliance or device.

Note, the steps, connections, and processing of signals and informationillustrated in the figures, including, but not limited to, any block andflow diagrams and message sequence charts, may typically be performed inthe same or in a different serial or parallel ordering and/or bydifferent components and/or processes, threads, etc., and/or overdifferent connections and be combined with other functions in otherembodiments, unless this disables the embodiment or a sequence isexplicitly or implicitly required (e.g., for a sequence of read thevalue, process said read value—the value must be obtained prior toprocessing it, although some of the associated processing may beperformed prior to, concurrently with, and/or after the read operation).Also note, nothing described or referenced in this document is admittedas prior art to this application unless explicitly so stated.

The term “one embodiment” is used herein to reference a particularembodiment, wherein each reference to “one embodiment” may refer to adifferent embodiment, and the use of the term repeatedly herein indescribing associated features, elements and/or limitations does notestablish a cumulative set of associated features, elements and/orlimitations that each and every embodiment must include, although anembodiment typically may include all these features, elements and/orlimitations. In addition, the terms “first,” “second,” etc. aretypically used herein to denote different units (e.g., a first element,a second element). The use of these terms herein does not necessarilyconnote an ordering such as one unit or event occurring or coming beforeanother, but rather provides a mechanism to distinguish betweenparticular units. Moreover, the phrases “based on x” and “in response tox” are used to indicate a minimum set of items “x” from which somethingis derived or caused, wherein “x” is extensible and does not necessarilydescribe a complete list of items on which the operation is performed,etc. Additionally, the phrase “coupled to” is used to indicate somelevel of direct or indirect connection between two elements or devices,with the coupling device or devices modifying or not modifying thecoupled signal or communicated information. Moreover, the term “or” isused herein to identify a selection of one or more, including all, ofthe conjunctive items. Additionally, the transitional term “comprising,”which is synonymous with “including,” “containing,” or “characterizedby,” is inclusive or open-ended and does not exclude additional,unrecited elements or method steps. Finally, the term “particularmachine,” when recited in a method claim for performing steps, refers toa particular machine within the 35 USC §101 machine statutory class.

Expressly turning to the figures, FIG. 1 illustrates network 100,including packet switching devices 101-106 arranged in a ring topology.Note, of course there can be additional packet switching devices in thering topology, and/or additional packet switching devices connected toone or more of packet switching devices 101-106.

As shown, particular packet switching device 101 is communicativelycoupled to neighbor packet switching device 106 via particularcommunications 111, which is initially functioning in our examplenetwork 100 of one embodiment. Additionally as shown, the other neighborof particular packet switching device 101 is second neighbor packetswitching device 102. Intermediate packet switching device 103 isconfigured (e.g., via a link state routing protocol) to forward packetsaddressed to neighbor packet switching device 106 towards packetswitching device 102 in the ring topology of network 100 (and thereforethrough particular packet switching devices 102 and 101 to neighborpacket switching device 106). Intermediate packet switching device 104is configured (e.g., via a link state routing protocol) to forwardpackets addressed to neighbor packet switching device 106 towards packetswitching device 105 in the ring topology of network 100 (and thereforethrough particular packet switching device 105 to neighbor packetswitching device 106).

In one embodiment, each of packet switching devices 101-106 operate alink-state routing protocol (e.g., Open Shortest Path First (OSPF) orIntermediate System to Intermediate System (IS-IS)) to understand theconnectivity of network 100, which is used to determine how to forwardpackets in network 100.

When particular communications 111 fails or is otherwise taken out ofservice (e.g., for maintenance purposes), data packets being forwardedto neighbor packet switching device 106 (and possibly to other packetswitching devices) through particular packet switching device 101 aretraveling around the ring topology of network 100 towards the failure asparticular packet switching device 101 is no longer communicativelycoupled to neighbor packet switching device 106 via particularcommunications 111. Hence, to reach neighbor packet switching device 106with particular communications 111 out of service, data packets shouldbe sent around the ring topology of network 100 in the direction awayfrom particular packet switching device 101/failed particularcommunications 111. However, the convergence of forwarding informationin network 100 is not immediate in response to a change in the topologyof network 100, such as in response to a failure of particularcommunications 111.

In response to this failure, a prior packet switching device (in theposition of packet switching device 101) would send out link stateinformation designating that particular communications 111 was no longeravailable, and this updated link state information would propagatearound network 100 in a counter-clockwise direction, with packetswitching devices serially updating their forwarding tables andforwarding the link state information. Prior systems communicate linkstate information in link state packets which are exchanged between onlyneighboring packet switching devices. Thus, for example, a prior firstpacket switching device in the position of packet switching device 103in network 100 could be still sending packets to a prior second packetswitching device in the position of packet switching device 102 innetwork 100, with the second packet switching device forwarding thesepackets back to the first packet switching device. This is called amicro-loop condition, as packets are being looped back to a previouslysending packet switching device.

To avoid micro-loops in one embodiment, particular packet switchingdevice 101 will communicate the link state information concerning theunavailability of particular communications 111 to a packet switchingdevice that is already configured to forward packets addressed toneighbor packet switching device 106, but not through particular packetswitching device 101. From there, this link state information concerningparticular communications 111 will normally propagate betweenneighboring packet switching devices through network 100. In thismanner, packet switching devices will typically be updated in an orderto avoid one or more micro-loops.

Further, packet switching device 101 of one embodiment advertises thatthis link state information as being a very large cost (e.g., maxCost—1)of the now unavailable particular communications 111, rather than beingin a failed condition which would be considered by a packet switchingdevice that particular communications 111 failed in both directions.Subsequently, such as in response to receipt of this link stateinformation or in response to the expiration of a time period, packetswitching device 101 will advertise that failure of this particularcommunications 111 via standard link state protocol message(s).

For example, particular packet switching device 101 recognizes thatparticular communications 111 is no longer available. In response,particular packet switching device 101 communicates link stateinformation, that the cost of particular communications 111 (for thedirection of packet switching device 101 to 106) to be maxCost-1, to anyof packet switching devices 104-106. Because each of packet switchingdevices 104-106 is configured to forward packets to packet switchingdevice 106 not through particular packet switching device 101, or ispacket switching device 106, the normal propagation of link stateinformation will not result in a micro-loop (as long as the packetswitching devices update their forwarding information before a nextpacket switching device updates its forwarding information concerningthe high cost of particular communications 111).

Similarly, neighbor packet switching device 106 recognizes thatparticular communications 111 is no longer available. In response,neighbor packet switching device 106 communicates link stateinformation, that the cost of particular communications 111 (for thedirection of packet switching device 106 to 101) to be maxCost-1, to anyof packet switching devices 101-103. Because each of packet switchingdevices 101-103 is configured to forward packets to packet switchingdevice 101 not through particular packet switching device 106, or ispacket switching device 101, the normal propagation of link stateinformation will not result in a micro-loop (as long as the packetswitching devices update their forwarding information before a nextpacket switching device updates its forwarding information concerningthe high cost of particular communications 111).

One embodiment of packet switching device 200 (e.g., appliance, corerouter, edge router) is illustrated in FIG. 2. As shown, packetswitching device 200 includes line cards 201 and 205, each with one ormore FIBs for use in forwarding packets.

Additionally, packet switching device 200 also has a route processor202, which typically manages the control plane by communicating routinginformation (e.g., exchanging link state information) with other packetswitching devices, populates one or more RIBs, and populates one or moreFIBs in line cards 201 and 205 for use in communicating packets. Packetswitching device 200 also includes other cards 204 (e.g., service cards,blades), and some communication mechanism 203 (e.g., bus, switchingfabric, matrix) for allowing its different entities 201, 202, 204 and205 to communicate.

FIG. 3 is a block diagram of an apparatus or component 300 used in oneembodiment. One embodiment includes one or more packet switchingdevices, and/or one or more packet switching devices configured tooperate, or actually operating, in a network. In one embodiment,apparatus or component 300 performs one or more processes correspondingto one of the flow diagrams illustrated or otherwise described herein,and/or illustrated in another diagram or otherwise described herein.

In one embodiment, apparatus or component 300 includes one or moreprocessing element(s) 301, memory 302 (e.g., one or more memories),storage device(s) 303, specialized component(s) 305 (e.g. optimizedhardware such as for converting addresses, performing operations, etc.),and interface(s) 307 for communicating information (e.g., sending andreceiving packets, user-interfaces, displaying information, etc.), whichare typically communicatively coupled via one or more communicationsmechanisms 309, with the communications paths typically tailored to meetthe needs of a particular application. In one embodiment, apparatus orcomponent 300 corresponds to, or is part of, a packet switching device101-106 of FIG. 1.

Various embodiments of apparatus or component 300 may include more orfewer elements. The operation of apparatus or component 300 is typicallycontrolled by processing element(s) 301 using memory 302 and storagedevice(s) 303 to perform one or more tasks or processes. Memory 302 isone type of computer-readable/computer-storage medium, and typicallycomprises random access memory (RAM), read only memory (ROM), flashmemory, integrated circuits, and/or other memory components. Memory 302typically stores computer-executable instructions to be executed byprocessing element(s) 301 and/or data which is manipulated by processingelement(s) 301 for implementing functionality in accordance with anembodiment. Storage device(s) 303 are another type of computer-readablemedium, and typically comprise solid state storage media, disk drives,diskettes, networked services, tape drives, and other storage devices.Storage device(s) 303 typically store computer-executable instructionsto be executed by processing element(s) 301 and/or data which ismanipulated by processing element(s) 301 for implementing functionalityin accordance with an embodiment.

Briefly, illustrated in FIG. 5 are generalizations of four differentpackets possibly used in one embodiment to communicate link stateinformation 501 between packet switching devices. A basic link statepacket 500 is shown which includes link state information 501. Also,shown is a tunnel/link state packet 510 which is used in one embodimentto tunnel link state information 501 to a packet switching device.Tunnel information 511 is used to represent information typicallyincluded in a header of a packet being tunneled, with tunnel informationbeing such as, but not limited to Multiprotocol Label Switching (MPLS)label(s), Internet Protocol (IP) address(es), and/or other forwardinginformation. Also, shown is a tunnel/destination/link state packet 520which is used in one embodiment to tunnel a packet including destinationinformation 521 and link state information 501 to a packet switchingdevice. This packet switching device will then send a destination/linkstate packet 530 including link state information 501 to a destinationpacket switching device identified by destination information 521 (e.g.,MPLS label(s), IP address, and/or other forwarding information, etc.).In one embodiment, destination/link state packet 530 is used tocommunicate link state information 501 from a particular packetswitching device to an intermediate packet switching device identifiedby destination information 521.

FIG. 4 illustrates a process configured to be performed, and/orperformed, in one embodiment. Processing begins with process block 400.Note, the process illustrated by FIG. 4 is performed by each particularpacket switching device of the packet switching devices on both sides ofthe particular communications which has failed or to be taken out ofservice.

In process block 402, the particular packet switching device identifies(e.g., receives a notification, detects a condition) that the particularcommunications to the neighbor packet switching device has failed or isabout to go down.

As determined in process block 403, if the particular communications isstill up (e.g., it will be taken down shortly), then, in process block404, the particular packet switching device directly communicates to,and only to, the neighbor packet switching device of the high cost ofthe particular communications (e.g., from the particular to the neighborpacket switching device). This link state information will thereforepropagate around the packet switching devices in the ring topology in aorder to avoid micro loops. FIG. 5 illustrates a link state packet 500used to communicate link state information 501 in one embodiment.

Otherwise as determined in process block 403 of FIG. 4, the particularcommunications has failed. In process block 406, the particular packetswitching device communicates link state information of a high cost ofthe particular communications (e.g., in the direction from particular toneighbor packet switching devices) such that it will propagate towardsthe particular packet switching device from at least from the furthestpacket switching device in the ring topology that is currentlyconfigured to forward packets having a destination address of theneighbor packet switching device not through the particular packetswitching device. In one embodiment, the particular packet switchingdevice uses a packet 510, 520 or 530 (of FIG. 5) to communicate thislink state information.

In one embodiment, the particular link state information is marked usinga type-length-value field (TLV) or via some other mechanism to informthe packet switching device receiving this link state information thatit corresponds to information to be updated in a manner to prevent orminimize micro-loops. Based on this marking, a packet switching devicemight delay the further flooding of this particular link stateinformation to another packet switching device until it has updated thecontents of its forwarding information to reflect the high cost of theparticular communications (e.g., link). If all packet switching devicesin the ring topology delay flooding until its respective forwardinginformation has been updated, this ensures that even where individualpacket switching devices process the information and update theirforwarding information at different speeds, the correct ordering of theforwarding information updating for minimizing or elimination microloops will be maintained.

As the particular packet switching device of one embodiment advertised ahigh-cost of the particular communications which is technically notcorrect information, in process block 408 of FIG. 4, the particularpacket switching device floods link state information that theparticular communications is unavailable (which propagates around thering topology and is the correct status of the particularcommunications) in response to receiving, or after a predeterminedduration after sending, the previously sent link state information. Inone embodiment, the particular packet switching device identifies thatit has received the previously sent link state information based on asequence number associated with the previously sent link stateinformation, or via some other mechanism. Processing of the flow diagramof FIG. 4 is complete as indicated by process block 409.

In view of the many possible embodiments to which the principles of thedisclosure may be applied, it will be appreciated that the embodimentsand aspects thereof described herein with respect to thedrawings/figures are only illustrative and should not be taken aslimiting the scope of the disclosure. For example, and as would beapparent to one skilled in the art, many of the process block operationscan be re-ordered to be performed before, after, or substantiallyconcurrent with other operations. Also, many different forms of datastructures could be used in various embodiments. The disclosure asdescribed herein contemplates all such embodiments as may come withinthe scope of the following claims and equivalents thereof.

What is claimed is:
 1. A network, comprising: a plurality of packetswitching devices communicatively coupled in a ring topology, with theplurality of packet switching devices including: a particular packetswitching device, a neighbor packet switching device, a second neighborpacket switching device, and an intermediate packet switching device;wherein the particular packet switching device and the neighbor packetswitching device are neighbors in the ring topology when particularcommunications is established between the particular packet switchingdevice and the neighbor packet switching device; wherein the particularpacket switching device and the second neighbor packet switching deviceare neighbors in the ring topology; wherein each of the plurality ofpacket switching devices are configured to exchange link stateinformation and to update their respective local forwarding informationaccordingly; and wherein the particular packet switching device isconfigured to cause a particular link state packet, including particularlink state information identifying a very high cost of the particularcommunications for reaching the neighbor packet switching device fromthe particular packet switching device, to be communicated to theneighbor packet switching device such that said particular link stateinformation is processed by the neighbor packet switching device beforeany other packet switching device in the plurality of packet switchingdevices; and wherein the particular packet switching device isconfigured not to flood said particular link state information to thesecond neighbor packet switching device.
 2. The network of claim1,wherein said configuration of the particular switching device includessaid causing of the particular link state packet to be communicated tothe neighbor packet switching device in response to the particularcommunications being no longer available between the particular packetswitching device and the neighbor packet switching device.
 3. Thenetwork of claim 1, wherein the intermediate packet switching device isconfigured to forward packets addressed to the neighbor packet switchingdevice to another packet switching device of the plurality of packetswitching devices towards the neighboring node and away from theparticular packet switching device in the ring topology; wherein thepacket switching device is configured to communicate with theintermediate packet switching device via a tunnel through one or morepacket switching devices of the plurality of packet switching devices;and wherein said causing of the particular link state packet to becommunicated to the neighbor packet switching device includes sending atunnel packet, including a packet addressed to the neighbor packetswitching device and including the particular link state packet, overthe tunnel.
 4. The network of claim 1, wherein the particular packetswitching device is configured to flood to the second neighbor packetswitching device, subsequent to said causing of the particular linkstate packet to be communicated to the neighbor packet switching device,link state information identifying the unavailability of the particularcommunications.
 5. The network of claim 4, wherein said flooding of saidlink state information identifying the unavailability of the particularcommunications is configured to be performed in response to theparticular packet switching device receiving the particular link stateinformation from the second neighbor packet switching device.
 6. Thenetwork of claim 4, wherein said flooding of said link state informationidentifying the unavailability of the particular communications isconfigured to be performed in response to an expiration of a time periodafter said causing of the particular link state packet to becommunicated to the neighbor packet switching device.
 7. The network ofclaim 1, wherein each packet switching device, different than theparticular packet switching device, of the plurality of packet switchingdevices is configured to flood said particular link state informationonly after updating its local forwarding information.
 8. A network,comprising: a plurality of packet switching devices communicativelycoupled in a ring topology, with the plurality of packet switchingdevices including: a particular packet switching device, a neighborpacket switching device, a second neighbor packet switching device, andan intermediate packet switching device; wherein the particular packetswitching device and the neighbor packet switching device are neighborsin the ring topology when particular communications is establishedbetween the particular packet switching device and the neighbor packetswitching device; wherein the particular packet switching device and thesecond neighbor packet switching device are neighbors in the ringtopology; wherein each of the plurality of packet switching devices areconfigured to exchange link state information and to update theirrespective local forwarding information accordingly; wherein theintermediate packet switching device is configured to forward packetsaddressed to the neighbor packet switching device to another packetswitching device of the plurality of packet switching devices towardsthe neighboring node and away from the particular packet switchingdevice in the ring topology, or is the furthest packet switching devicefrom the particular packet switching device in the ring topology that isconfigured to forward packets addressed to the neighbor packet switchingdevice through the particular packet switching device; and wherein theparticular packet switching device is configured to encapsulate aparticular link state packet, including particular link stateinformation identifying a very high cost of the particularcommunications for reaching the neighbor packet switching device fromthe particular packet switching device, and to send said encapsulatedparticular link state packet to the intermediate packet switching devicein response to the particular communications being no longer availablebetween the particular packet switching device and the neighbor packetswitching device; and wherein the particular packet switching device isconfigured not to flood said particular link state information to thesecond neighbor packet switching device.
 9. The network of claim 8,wherein the intermediate packet switching device is configured toprocess said particular link state information, and to flood saidparticular link state information to a next packet switching device ofthe plurality of packet switching devices towards the particular packetswitching device.
 10. The network of claim 9, wherein the intermediatepacket switching device is configured to flood said particular linkstate information to a next packet switching device of the plurality ofpacket switching devices towards the neighbor packet switching device.11. The network of claim 8, wherein said encapsulated particular linkstate packet is communicated over a tunnel between the particular packetswitching device and the intermediate packet switching device.
 12. Thenetwork of claim 8, wherein the particular packet switching device isconfigured to flood to the second neighbor packet switching device,subsequent to said sending said encapsulated particular link statepacket to the intermediate packet switching device, link stateinformation identifying the unavailability of the particularcommunications.
 13. The network of claim 12, wherein said flooding ofsaid link state information identifying the unavailability of theparticular communications is configured to be performed in response tothe particular packet switching device receiving the particular linkstate information from the second neighbor packet switching device. 14.The network of claim 12, wherein said flooding of said link stateinformation identifying the unavailability of the particularcommunications is configured to be performed in response to anexpiration of a time period after said sending said encapsulatedparticular link state packet to the intermediate packet switchingdevice.
 15. A method, comprising: performing operations by a particularpacket switching device in a network; wherein the network includes aplurality of packet switching devices communicatively coupled in a ringtopology, with the plurality of packet switching devices including: theparticular packet switching device, a neighbor packet switching device,a second neighbor packet switching device, and an intermediate packetswitching device; wherein the particular packet switching device and theneighbor packet switching device are neighbors in the ring topology whenparticular communications is established between the particular packetswitching device and the neighbor packet switching device; wherein theparticular packet switching device and the second neighbor packetswitching device are neighbors in the ring topology; wherein each of theplurality of packet switching devices are configured to exchange linkstate information and to update their respective local forwardinginformation accordingly; and wherein said operations include: generatinga particular link state packet, including particular link stateinformation identifying a very high cost of the particularcommunications for reaching the neighbor packet switching device fromthe particular packet switching device; and communicating the particularlink state packet to the neighbor packet switching device such that saidparticular link state information is processed by the neighbor packetswitching device before any other packet switching device in theplurality of packet switching devices, but not flooding said particularlink state information to the second neighbor packet switching device.16. The method of claim 15, wherein said operation of generating theparticular link state packet is performed in response to the particularcommunications being no longer available between the particular packetswitching device and the neighbor packet switching device.
 17. Themethod of claim 16, wherein said operations include flooding to thesecond neighbor packet switching device, subsequent to said causing ofthe particular link state packet to be communicated to the neighborpacket switching device, link state information identifying theunavailability of the particular communications.
 18. The method of claim15, wherein the intermediate packet switching device is configured toforward packets addressed to the neighbor packet switching device toanother packet switching device of the plurality of packet switchingdevices towards the neighboring node and away from the particular packetswitching device in the ring topology; and wherein said operation ofcommunicating the particular link state packet to the neighbor packetswitching device includes sending a tunnel packet, including a packetaddressed to the neighbor packet switching device and including theparticular link state packet, over a tunnel through one or more packetswitching devices of the plurality of packet switching devices to theintermediate packet switching device.
 19. A method, comprising:performing operations by a particular packet switching device in anetwork; wherein the network includes a plurality of packet switchingdevices communicatively coupled in a ring topology, with the pluralityof packet switching devices including: the particular packet switchingdevice, a neighbor packet switching device, a second neighbor packetswitching device, and an intermediate packet switching device; whereinthe particular packet switching device and the neighbor packet switchingdevice are neighbors in the ring topology when particular communicationsis established between the particular packet switching device and theneighbor packet switching device; wherein the particular packetswitching device and the second neighbor packet switching device areneighbors in the ring topology; wherein each of the plurality of packetswitching devices are configured to exchange link state information andto update their respective local forwarding information accordingly; andwherein said operations include: encapsulating a particular link statepacket, including particular link state information identifying a veryhigh cost of the particular communications for reaching the neighborpacket switching device from the particular packet switching device; andsending said encapsulated particular link state packet to theintermediate packet switching device in response to the particularcommunications being no longer available between the particular packetswitching device and the neighbor packet switching device; not floodingsaid particular link state information to the second neighbor packetswitching device.
 20. The method of claim 19, comprising: processing, bythe intermediate packet switching device, said particular link stateinformation to update forwarding information of the intermediate packetswitching device; and flooding said particular link state information toa next packet switching device of the plurality of packet switchingdevices towards the particular packet switching device, and to a nextpacket switching device of the plurality of packet switching devicestowards the neighbor packet switching device.
 21. The method of claim20, wherein the intermediate packet switching device is configured toforward packets addressed to the neighbor packet switching device toanother packet switching device of the plurality of packet switchingdevices towards the neighboring node and away from the particular packetswitching device in the ring topology, or is the furthest packetswitching device from the particular packet switching device in the ringtopology that is configured to forward packets addressed to the neighborpacket switching device through the particular packet switching device.22. The method of claim 21, wherein said operations include flooding tothe second neighbor packet switching device, subsequent to said causingof the particular link state packet to be communicated to the neighborpacket switching device, link state information identifying theunavailability of the particular communications.